1. Field of the Invention
The invention relates to the membrane gas separations. More particularly, it relates to membrane dryers using a purge gas to enhance the drying operation.
2. Description of the Prior Art
There are many commercial circumstances in which it is necessary or desirable to remove moisture from a gas stream. Water vapor is a common impurity in many raw or process gases, and it often acts as a contaminant or corrosive agent that must be removed, or reduced in concentration, before the gas can be used. For example, dry air is often required in pneumatic systems, such as the instrument air used in chemical processing plants. Gases that are used for inert atmospheres must also be highly dried, since residual water vapor can be reactive rather than inert. In other instances, contained moisture can condense or freeze, thereby inhibiting the flow of process streams. Effective means for drying gas streams are, therefore, needed in the art.
Many different means have been customarily employed for drying gas streams. In some cases, mere compression of the gas is sufficient to condense the water vapor to liquid, which can be drained away from the gas stream. Likewise, chillers and cryogenic traps can condense and remove the water as a liquid or as solid ice. Condensation methods are very useful for some applications, but they are often inadequate when a very dry gas stream is required.
Adsorption processes are also often employed for gas drying purposes, since many adsorbents have a strong adsorptive affinity for water. Such adsorbents soon become saturated, however, and must be regenerated periodically if the drying process is to operate continuously. In pressure swing adsorption (PSA) processing, the adsorption is carried out at an upper adsorption pressure. Some of the dry product gas is depressurized and used as a countercurrent purge stream to facilitate desorption of water from the adsorbent bed at a lower desorption pressure. This PSA process can produce very dry gas streams, but some of the product gas must necessarily be recycled for such purge gas purposes and discharged from the system as a waste gas.
Membrane permeation is a particularly attractive drying approach, offering certain advantages over other drying means. It is well known that water vapor is very highly permeable in many synthetic polymer membranes. When a moisture-laden gas is passed over such a membrane, the water vapor will tend to penetrate the membrane and pass through it from the feed to the permeate side provided that a sufficient drying force is present to facilitate the permeation of the water vapor through the membrane. For a commercially suitable drying process, the gas to be dried must be exposed to a large surface area of membrane that is very thin so that the diffusion path in the membrane material is very short. A pressure differential must also be maintained across the membrance to provide the drawing force for a suitable permeation action. In addition, a flow pattern must be established that enables the gas stream being processed to be progressively exposed to additional membrane surface so that the remaining moisture in the gas stream can continue to permeate and be removed from the membrane system. Such processes can conveniently be carried out in a permeation module comprising a large number of so-called composite or asymmetric hollow fibers. Such permeation modules are well known in the art and are becoming widely used for an increasingly broad range of commercial gas separation operations.
It has been determined that so-called 3-port permeators have genuine limitations when used for the drying of low-permeability gases. Such 3-port permeations have a feed gas inlet port and separate outlet ports for the permeate and non-permeate portions of the feed gas. Although the water is highly permeable, it can be effectively removed from the low-pressure passages of the membrane only when there is sufficient permeation of other gases. To act as effective dryers, such permeators must operate with a high stage-cut, which means that a considerable amount of the gas being dried must also be permeated, and thus lost as dry gas product. It has been determined that improved drying can be achieved when a 4-port permeator is employed, provided that there is a high degree of radial mixing within the hollow fibers. With the 4-port permeator, a separate dry purge stream is introduced through the fourth port for passage on the permeate side of the hollow fibers, thereby flushing moisture from the low-pressure passages of the fibers. As a result, purge drying is found to be more effective than permeation drying. Even when dry product gas is used for purge purposes, the purge drying process is superior because forcing the permeation of product gas requires a high pressure difference or a large membrane surface area, both of which are unnecessary when a separate purge gas is used.
Prasad, U.S. Pat. No. 4,931,070 describes the use of a 4-port membrane module, operated in a countercurrent flow pattern, as a gas dryer. In particular, this reference relates to the production of nitrogen, wherein feed air is passed through two membrane permeator stages, wherein the bulk of the oxygen in the air is removed from the nitrogen. The residual oxygen impurity is removed by catalytic reaction with hydrogen in a "deoxo" unit. The water generated by this reaction is largely removed by passing the wet nitrogen gas through a cooler and liquid water separator. Nevertheless, a substantial amount of water impurity remains in the thus-processed nitrogen stream. This residual moisture is removed by a membrane dryer, the low pressure passages on the permeate side thereof being purged by air, the dry permeate from the second stage membrane or by dry nitrogen product.
Despite such advantageous drying processes, further improvements are desired in the art to enhance the membrane drying of gases in practical commercial operations. When product gas is used for purging, a certain amount of product gas is, of course, lost in the purge waste stream. When an external source of dry purge is used some undesired contamination of the product stream can occur by back diffusion of some of the non-product components present in the purge gas. These factors create a practical limitation on the ultimate usefullness of the membrane drying methods referred to above.
It is an object of the invention to provide an improved membrane process and system for the separation of gases.
It is another object of the invention to provide a membrane gas separation drying process and system wherein the back diffusion of impurities from an external source of purge gas is minimized.
It is another object of the invention to provide an improved membrane drying process and system wherein the amount of product gas or external purge gas used as purge is minimized.
It is a further object of the invention to provide an enhanced process and system for removing water vapor from high purity nitrogen without recontamination of product nitrogen during drying and with a high degree of product nitrogen recovery.
With these and other objects in mind, the invention is hereafter described in detail, the novel features thereof being particularly pointed out in the appended claims.